The present invention relates to a switching device for generating a switch signal, which is equally suitable for the integration into a transponder reading device and for subsequently equipping a transponder reading device. Furthermore, the invention relates to a reading device having such a switching device and a method for the communication between a reading device and a transponder.
B. Related Art
In standards ISO/IEC 14443 and ISO/IEC 15693 a method is described, how a transponder reading device can set up a data connection with a transponder of another device (contactless chip card/RFID transponder). For this purpose the reading device periodically emits a search signal with a high field strength (e.g. 1.5-7.5 A/m according to ISO/IEC 14443) until a device provided with transponder reaches the response range of the reading device, is provided with energy from the field of the reading device and sends a response to the reading device. For reading devices with limited energy resources, however, this results in a reduction of the possible operating lifetime, in particular for battery powered devices.
In order to increase the operating lifetime of a reading device, U.S. Pat. No. 6,150,948 describes an RFID reading device, which with the help of an energy-saving detector searches for transponders in the surrounding. For example, this detector is a detector operating on an infrared basis. The RFID reading device itself is not activated until a transponder is detected in the proximity of the reading device.
From WO 03/069538 A1 is known a switching device for a reading device to be activated with the help of a transponder, which as long as a switching operation is not triggered can be operated in a nearly loadless fashion. For this purpose the reading device to be activated is provided with a coil, which is part of an oscillating circuit, which in the recognition mode is operated as a substantially unloaded pure oscillating circuit. The resonant frequency in the oscillating circuit is monitored by a frequency observer. If a transponder with a transponder coil approaches the recognition coil, the resonant frequency of the oscillating circuit changes. This is detected by the frequency observer, which thereupon generates a switch signal that switches on the reading device to be activated. With that it is possible to recognize a transponder in the proximity of the coil even without a transmitting power (worth mentioning), since the field strength of the search signal emitted during the—optionally periodical—measuring operation no longer is used for supplying a transponder with energy.
WO 2005/013506 A1, too, describes a measuring and switching device for measuring the influence an approaching transponder has on a transmitting antenna of an RFID reading device. WO 2005/013506 A1 in particular describes in detail the processes in a reading device for detecting a transponder without the emission of energy for supplying this transponder with energy. A substantial part of WO 2005/013506 A1 is a measuring device for measuring the electrical properties of the transmitting antenna of the reading device, the circuit of the measuring device for example being formed as a PLL circuit. PLL stands for “phase locked loop” and means, that a signal with a frequency is set so exactly relative to a signal with a reference frequency that the phase relation between the signals is permanently set. A disadvantage of the described PLL circuit is, however, that it uses the antenna coil of the reading device antenna as a frequency-determining component of an oscillator. This means that the PLL circuit must be directly connected with the antenna coil. But with many reading devices the reader antenna is adapted with an adapter circuit to an impedance of 50Ω and is installed via a coaxial cable connection, detached from the electronics of the reading device. With such an arrangement the described PLL circuit can be realized only with difficulties or cannot be realized at all.
Therefore, it is the problem of the present invention to specify a switching device for a transponder reading device to be activated to be actuated by a transponder which on the side of the reading device to be activated has a power consumption as low as possible and, furthermore, in terms of circuitry is especially easy to realize.
This problem is solved by a switching device, a reading device provided therewith and a method according to the independent claims. In claims dependent thereon advantageous embodiments and developments of the invention are specified.